1. Field of the Invention
The present invention relates to a wireless communication system which adjusts frame timing between base stations, a base station and a management server used in the wireless communication system, and a wireless communication method.
2. Description of the Related Art
A wireless communication system such as a Personal Handy-phone System (PHS) or mobile phones employs a TDMA/TDD scheme which, while using Time Division Duplex (TDD) of dividing a communication path with respect to a time axis and alternately transmitting and receiving data through the divided communication paths to realize a concurrent bidirectional communication, uses a Time Division Multiple Access (TDMA) scheme of dividing the transmitting and receiving times (frames) into a plurality of time slots. It is considered that an Association of Radio Industries and Business (ARIB) STD T95 or PHS Memorandum of Understanding (MoU) is employed as a next-generation PHS communication standard enabling a high-speed digital communication.
In the communication standard such as the ARIB STD T95 or the PHS MoU, an Orthogonal Frequency Division Multiplexing (ODFM) scheme is employed. The OFDM scheme is one of multiplexing schemes effectively using a plurality of carrier waves in a unit time axis by allowing the frequency bands of the carrier waves to partially overlap with each other so that signal waves to be modulated are orthogonal in phase between the neighboring carrier waves. An Orthogonal Frequency Division Multiplex Access (OFDMA) is also provided in which a plurality of users share all subchannels and a subchannel having the best transmission efficiency is assigned to the corresponding user while subchannels are assigned to individual users in a time division manner in the OFDM scheme.
In any of the above-mentioned communication schemes, a mobile station sets up a communication link with a base station through a channel or subchannel assigned to a base station. Accordingly, the mobile station has to be synchronous with frame timing (transmitting and receiving times) of data in the base station. The base station broadcasts a control signal (control signal transmission channel (CCH: Control Channel)), for example, every 20 frame timings (5 millisecond), that is, every 100 millisecond, and the mobile station receives the control signal and sets up a wireless communication in synchronization with the control signal.
The frame timing synchronization (frame synchronization) has to be made between base stations, as well as between a mobile station and a base station. In a wireless communication system, a plurality of base stations wirelessly communicate with a plurality of mobile stations using the same wireless frequency bands. Accordingly, when the base stations set up wireless communications with the mobile stations at frame timings independent from each other, electric wave interference or fading may occur.
It is considered that a handover of changing a communicating base station of a mobile station is carried out due to a change in electric wave between the mobile station and the base station. When the frame timings of the base stations are different from each other, radio signals cannot be effectively acquired so disable a smooth handover. Therefore, the frame synchronization between the base stations has to be carried out so that the plurality of base stations have the same frame timing before carrying out the synchronization between the mobile station and the base station. The frame synchronization is carried out to be synchronous with the absolute time acquired by a Global Positioning System (GPS) in a master base station, and is carried out in a slave station to be synchronous with a control signal of the master base station or another slave base station. When the frame synchronization is carried out in this way, it is possible to avoid interference with different slots in neighboring base stations.
When different base stations use the same frequency band in the same slot, a problem in the similar interference may occur. As described in JP-A-2005-027189 or US2005/0037763, this problem can be avoided using a technique of grouping a plurality of cells into the number of groups smaller than the number of cells constituting a closed cluster and assigning frequency bands to the groups. Additionally, JP-A-2004-508744 or U.S. Pat. No. 6,751,206 describes a technique to reduce interference due to transmission to a mobile station in a sector of a cell and its neighboring cells, by using a beam switching technique.
Interference can be theoretically avoided by the above-described frame synchronization or the assignment of proper frequency bands in the same slot. However, it may be actually influenced by electric waves from a base station apart by an assumed distance or more. In this case, even when the frame timings are completely synchronized in the absolute time, a propagation delay occurs due to the distance. When the propagation delay exceeds a guard interval (GI) provided for compensating the electric wave interference or fading, inter-symbol interference may be caused.
In base stations apart from each other, a control signal which is periodically broadcasted causes constant inter-symbol interference (ISI). In the wireless communication system employing the OFDMA, when subchannels including all subcarriers or a plurality of subcarriers of one base station are set to high power according to a high modulation class, the other base station cannot make a communication through all the subcarriers or subchannels due to the inter-symbol interference.
When the frame timing of one base station is deviated due to a disorder or defect after the frame synchronization has been completed between neighboring base stations, the electric waves from the base station may cause the inter-symbol interference in the other base station, similarly to the base stations apart by a large distance.